Pitched Roofs
Pitched roofs clad with tiles or slates remain an extremely
popular method for roofing many types of commercial and public
building.
Overview
The flexibility offered by this system of roofing reinforces
the comparatively low maintenance requirement of most pitched roofs
whilst also offering many options for creating attractive and
interesting exteriors.
Many designers of commercial and public buildings (especially
offices and schools) use the height offered in the pitch of the
roof to create light, attractive interiors with high level sloping
ceilings which include light wells and roof windows. The thermal
efficiency required by the Building Regulations, coupled with the
desire to create interesting and attractive interiors, can make the
specification of insulation for this application a complex
issue.
This page and its sub-pages provide solutions for both sloping
ceiling and flat ceiling applications.
Summary
Advantages
Pitched roofs can be insulated to create either warm
or cold roof spaces.
Insulation placed at flat ceiling level creates a cold roof space.
This is the lowest cost option for insulating a roof and uses
economical readily available mineral wool quilt insulation. Very
high levels of thermal insulation can be achieved because there is
virtually no restriction on the thickness of insulation that can be
installed.
Insulation placed in the plane of the roof pitch creates a warm
roof space. This option allows the designer to omit a flat ceiling
and use the space within the roof volume as extra accommodation
enhancing the usable area of the property without significantly
altering the building envelope.
Knauf Insulation
Products
- Crown Rafter Roll 32 is a high
performance roll of glass mineral wool. It is easy to cut and
friction fit between irregularly spaced rafters.
- Rocksilk Pitched Roof Slab is a high
density, water repellent, rock mineral wool slab specifically
developed for over rafter (sarking) insulation.
- Polyfoam Sarking Board System
is a high performance, 100% ozone friendly, extruded polystyrene,
rigid board insulation system. It is lightweight and consists of a
75mm ‘top
- Polyfoam Raftersqueeze is a high
performance, 100% ozone friendly, extruded polystyrene, rigid board
insulation. It consists of a flexible infill piece that fits
between rafters, and allows for some variation in timber rafter
centres.
- Polyfoam Pitched Roofboard is a
high performance, 100% ozone friendly, extruded polystyrene, rigid
board insulation system. It is lightweight with shiplap edge
details to sit above rafters in pitched roofs.
- Polyfoam Linerboard is a
laminate of Polyfoam high performance, 100% ozone friendly,
extruded polystyrene, rigid board insulation and 9.5mm tapered edge
plasterboard – to underdraw the interior rafter line.
- Crown Loft Roll 44 is made from
glass mineral wool and formed into unfaced rolls which are
lightweight, flexible, resilient and non-combustible – for ceiling
level insulation.
- Knauf Breatheline is a
breathable membrane with a polypropylene non-woven coating on both
sides.
- Rocksilk Flexible Slab is a
multi-purpose rock mineral wool slab that can be used in pitched
roofs between the rafters.
Detailed Design Considerations
Utilising the roof void
If part of the roof void is to be utilised it is most likely
that insulation will be placed at rafter level. Traditionally
insulation is placed between the rafters from below, leaving an air
gap between the insulation and the sarking felt for ventilation.
Finally, the ceiling is lined with a vapour control layer and
plasterboard.
With the high levels of thermal
performance demanded by the latest Building Regulations, it is
necessary to examine other solutions to ensure that the required
performance is met at reasonable cost.
The minimum structural rafter
depth, the roof pitch and the thickness of insulation needed to
meet the desired thermal performance all need to be considered in
designing the roof insulation solution.
Building Regulations
Typical U-value requirements for pitched roofs is shown
in the table below.
| |
U-Value
(W/m2K) - Rafter Level |
U-Value
(W/m2K) - Ceiling Level |
|
| England, Wales &
NI |
0.20 |
0.16 |
|
| Scotland |
0.20 |
0.16 |
|
| Ireland |
0.20 |
0.16 |
|
Controlling the condensation
risk
Where insulation is placed in
roofs there is a risk that condensation will form on surfaces on
the cold side of the insulation.
The main way of preventing this
is to install a vapour control layer on the warm side of the
insulation. This limits the amount of water vapour emitted from the
building that can enter the insulation layer.
The vapour control layer must be
continuous, well sealed at joints and should preferably be placed
behind services, such as electrical cables, to avoid
puncturing.
Where a vapour permeable
membrane, such as Knauf Breatheline, is used as the tiling
underlay, any water vapour that does pass through the insulation
layer can disperse through the tiling underlay to the outside air.
This is known as a ‘Breathing’ roof and is recommended by Knauf
Insulation for new buildings.
Where a traditional bitumen based
sarking felt, or other impermeable underlay, is used as the tiling
underlay, it is necessary to provide cross ventilation to the cold
side of the insulation to enable water vapour to dissipate to the
atmosphere and thus prevent condensation forming in the roof
construction.
Building Regulations set out
adequate levels of ventilation to avoid the risk of condensation
within the roof space. Approved Document F Ventilation gives
specific guidance, as does Part G of the Building Standards
(Scotland) Regulations. Both allow
BS 5250: 2002 to be used as an
alternative.
Position of insulation
There is a choice of insulation position in pitched
roofs:
- At rafter level
- Between the rafters
- Above and between the rafters
- Below and between the rafters
- At ceiling level
Between Rafter Insulation
Insulation between the rafters can be designed in two
ways:
- ‘breathing’ roof with vapour permeable underlay as tiling
underlay
- ventilated design
Breathing roof design
Insulation fully fills the rafter
space without an airspace between the insulation and tiling
underlay, which must be vapour permeable. If a thin layer of
insulation is installed, it is recommended that an
insulation/plasterboard laminate is used as the internal lining to
prevent thermal bridging.
A combined airtight/vapour
control layer should be placed on the warm side of the insulation.
This not only makes the ceiling convection tight, but also
restricts the amount of water vapour passing through the
ceiling.
Where cables and piped services are to be installed, the
plasterboard lining may be battened out to provide a suitable duct.
The services should be routed on the inside of the vapour control
layer to avoid any puncturing.
|
‘Breathing’ Roof
With a breathing roof, moisture vapour from the building can easily
diffuse through the vapour permeable underlay, eliminating the need
for cross ventilation.This has several advantages:
- the ‘vapour open’ construction avoids the risk of condensation
within the roof timbers
- the vapour permeable underlay also acts as a windproof layer on
the outside of the rafters
- the avoidance of draughts, improves the energy efficiency of
the roof construction.
Knauf Insulation recommend the use of breathing roof
construction for all pitched roofs.
|
Ventilated design
With this design, a 50mm ventilated
airspace must be provided between the insulation and the
tiling
underlay. Should the rafter depth be insufficient to
accommodate both the required thickness of insulation and the 50mm
ventilated airspace, an insulated dry lining is recommended. This
has the added benefit of minimising thermal bridging.
Ventilation openings should be provided at each and every roof void
at both low and high level. At the eaves, ventilation openings
should be equivalent to a 25mm continuous gap. At the ridge the
ventilation opening should be the equivalent of a 5mm continuous
gap each side of the ridge.
A vapour control layer must be applied to the warm side of the
insulation.
Insulation Below Rafters
In each case the insulation performance provided by fully filling
the rafters can be improved by the use of insulated plasterboard to
the ceiling.
Over Rafter Insulation
Insulation over the rafters provides a completely insulated
external envelope to the roof construction, and allows the maximum
usable space within the roof enclosure.
The insulation boards are laid over the rafters. It is important
that there should be no gaps in the insulation layer and no
ventilation between the outside and the roof space. A vapour
permeable underlay must be provided to direct wind driven rain and
snow to the gutter.
Counterbattens are fixed through the insulation boards and vapour
permeable underlay into the rafters. Where the over rafter
insulation is over 50mm thick, consideration should be given to the
fixing of the counter battens.
Further insulation will be required between rafters to meet
Building Regulation requirements. If the insulation board over the
rafters has a high vapour resistance, the use of mineral wool
between the rafters is not recommended unless the designer is
confident that an effective vapour control layer can be provided to
the underside of the rafters.
Where only rigid insulation boards with a high vapour resistance
are used, such as extruded polystyrene, a vapour control membrane
is not normally required unless the building is liable to high
humidity. An airspace is not required between the insulation and
tiling underlay if the latter is a vapour permeable underlay.
The rafters should be faced with 12.5mm plasterboard, or similar,
to provide fire protection.
Ceiling Level Insulation
This is the most common and economic
way to insulate a pitched roof and creates a ‘cold roof’. It has
the advantage of minimising both the heated volume and the quantity
of insulation material needed.
Cold ‘breathing’ roof
The features of a cold breathing roof are:
- a vapour control layer at ceiling level to limit the amount of
water vapour entering the roof void
- a vapour permeable underlay as the roof underlay, to allow
dissipation of water vapour from the roof void and, just as
importantly, to act as a windproof layer
- 25mm deep counterbattens to provide an airspace above the
vapour permeable underlay for dissipation of moisture vapour
- ceiling level insulation that is pushed up tight against the
vapour permeable underlay to prevent air leakage into the roof void
at eaves level Insulation is laid at ceiling level in two layers,
the first layer between the joists and the second layer across the
joists.
Cold ventilated roof
With the ventilated design, the insulation is also placed at
ceiling level, but cross ventilation of the roof void is used to
disperse moisture vapour.
It is important to allow a clear, unobstructed ventilation path
between the insulation and the tiling underlay to allow outside air
to enter the roof void and prevent the build-up of moisture.
The ventilation requirements for roofs with a double pitch are
shown above. Lean-to and mono-pitch roofs should have a continuous
gap equivalent to 10mm at the eaves and high level or ridge
ventilation equivalent to a continuous gap of 5mm.
Ventilation openings should prevent the entry of insects. A 3mm or
4mm mesh across the ventilation holes should be incorporated. A
separate vapour control layer is not necessary, but all gaps and
holes in the ceiling should be sealed to restrict the amount of
water vapour that enters the roof void.
Cold Roof – Other Precautions
All tanks and pipes in the roof void should also be insulated to
prevent freezing. Tank insulation jackets should meet the standards
in BS 5422: 2001. Do not insulate directly under the cold water
tank.
The roof access hatch should also be insulated to a minimum depth
of 50mm and draught stripped. Provide catches or bolts to compress
the draft seal and prevent air leakage from wind uplift.
Upgrading an Existing Pitched
Roof
An overall insulation thickness of 250mm to 300mm of mineral wool
is recommended when upgrading the thermal insulation of an existing
roof.
In most pitched roofs, any existing insulation is likely to be
between the ceiling joists. Where the existing insulation reaches
the top of the joists, the new insulation should simply be laid
across the joists at right angles. Where the existing insulation is
below the joist height, add extra insulation between the joists so
that it reaches the top of the joists. Then lay a second layer at
right angles to the joists.
Existing roofs are most likely to have an impervious sarking
underlay, such as a bitumen felt, so it is very important to
maintain the cross ventilation of the roof void from eaves to
eaves.
Thermal Bridges
To avoid thermal bridging, the roof insulation should butt up to or
lap the wall insulation. The designer should consider at what stage
this ‘linking’ insulation is installed, as this will affect the
detailing of insulation at the eaves.
In cold roofs, one way of achieving this in practice is to place a
short length of insulation quilt over the wall plate (and cavity
closer, where applicable) immediately before the tiling underlay is
fixed. This avoids having to push the insulation into place from
inside the roof once the roof covering has been completed.
The gap between gable/separating walls and the first joist/rafter
should be insulated to avoid thermal bridging. In cold roofs, the
second layer of insulation should be butted up against the gable
and separating walls to avoid thermal bridging.
Knauf Insulation products are supplied in widths to suit standard
joist/rafter spacings.
Recessed Light Fittings
Where recessed light fittings are to be used, specify ones with
compact fluorescent or low voltage tungsten lamps to minimise heat
build up. Locate the fittings in enclosures that provide at least
75mm clearance around the fitting for air to circulate. Seal the
enclosure to prevent air leakage into the roof void and, if
necessary, ventilate to the room.
Installation
Contractors should follow the guidance
given in HSE 33 when using and installing fragile materials.
Typical Details: Warm 'Breathing' Roof
Typical Specification Clauses
1) Mineral wool between rafters
The whole area of the pitched roof to be insulated with Crown
Rafter Roll 32 ......mm thick, in two layers, friction fitted
between the rafters. The insulation to be pushed over the wall
plate at the eaves to link up with the wall insulation.
A minimum 50mm airpath must be maintained between the insulation
and the sarking felt.
Alternatively, refer to NBS clauses: P10/140 and 320
2a) Insulation between rafters with Crown Rafter
Roll 32
The whole area of the pitched roof to be insulated with Crown
Rafter Roll 32, ......mm thick, friction fitted between the
rafters. The width of the insulation should be appropriate to the
spacing of the rafters.
Polyfoam Linerboard of thickness ......mm with integral 9.5mm
plasterboard nailed to the rafters at maximum 300mm centres and
finished using standard drylining techniques.
Alternatively, refer to NBS clauses: P10/140 and 320, and
K10/245
2b)Insulation between rafters with Rocksilk
Flexible Slab
The whole area of the pitched roof to be insulated with Rocksilk
Flexible Slab ......mm thick, friction fitted between the
rafters.
Polyfoam Linerboard of thickness ......mm with integral 9.5mm
plasterboard nailed to the rafters at maximum 300mm centres and
finished using standard drylining techniques.
Alternatively, refer to NBS clauses: P10/140 and 320, and
K10/245
3a) Rocksilk Pitched Roof Slab and Crown Rafter
Roll 32
Insulation laid over rafters to be
Rocksilk Pitched Roof Slab, ......mm thick, located over the
rafter. All boards tightly butted and with staggered joints. Knauf
Breatheline is then pulled taught and laid directly over the
insulation and held in place by 50 x 32mm counter battens nailed
through the slabs, into the rafters. The tiling battens to be
nailed into the counter battens.
Crown Rafter Roll 32 ......mm thick, to be friction fitted between
the rafters. The width of the insulation to be cut from the roll to
suit the spacing of the rafters.
Alternatively, refer to NBS clauses: K11/670 and P10/140 and
320
3b) Polyfoam Sarking Board
System
Insulation laid over rafters to be Polyfoam Sarking Board,
......mm thick, located over the rafter via the rebated edges.
Knauf Breatheline is then pulled taught and laid directly over the
insulation and held in place by 50 x 32mm counter battens nailed
through the boards, into the rafters. The tiling battens to be
nailed into the counter battens.
Alternatively, refer to NBS clauses: K11/670 and P10/140 and
320
3c) Polyfoam Pitched Roofboard and Polyfoam
Raftersqueeze
Insulation laid over rafters to be Polyfoam Pitched Roofboard,
......mm thick, located over the rafter. Knauf Breatheline is then
pulled taught and laid directly over the insulation and held in
place by 50 x 32mm counter battens nailed through the boards, into
the rafters. The tiling battens to be nailed into the counter
battens.
Insulation between the rafters to be Polyfoam Raftersqueeze,
supported on battens nailed to the side of the rafters, where
required.
Alternatively, refer to NBS clause: K11/670 and P10/140 and
320
4a) Ventilated cold roof
The whole area of the ceiling to be insulated with two layers of
Crown Loft Roll of total thickness ......mm. Width of first layer
of insulation to be appropriate to the joist spacings and of a
thickness equal to joist depth. The insulation to be laid between
the joists and to finish over wall plate on external walls. The
second layer to be laid at right angles to the first layer. All
joints between the rolls of insulation to be close butted.
Maintain an airspace between insulation and sarking felt/sarking
board at the eaves.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be insulated separately.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clauses: P10/120 and 130
4b) Cold Breathing Roof
The whole area of the ceiling to be
insulated with two layers of Crown Loft Roll of total thickness
......mm. Width of first layer of insulation to be appropriate to
the joist spacings and of a thickness equal to joist depth. The
insulation to be laid between the joists and to finish over wall
plate on external walls and pushed up tight to Knauf Breatheline.
The second layer of Crown Loft Roll to be laid at right angles to
the first layer.
All joints between the rolls of insulation to be close
butted.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be insulated separately.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clauses: P10/120, 130 and 320
4c) Upgrading Existing Insulation
The two alternative specifications:
a) Between joists (where applicable)
The whole area of the existing ceiling to be insulated with
Crown Loft Roll, thickness ......mm. Insulation of width
appropriate to the joist spacings to laid over the existing loft
insulation and dressed over wall plate.
b) Across the joists
The whole area of the existing ceiling to be insulated with
Crown Loft Roll, thickness ......mm. To be laid over the joists,
with all lateral joints close butted.
For both methods all joints to be close butted. Maintain airspace
between insulation and sarking felt/sarking board at eaves.
Do not insulate under cold water tanks. Cold water tanks and pipes
to be insulated separately.
Loft hatch to be insulated with a minimum 100mm depth of
insulation.
Alternatively, refer to NBS clause: P10/120 and 130
4d) Cold roof with storage deck
The whole area of the ceiling to be insulated with Crown Loft
Roll. Width of Crown Loft Roll to be appropriate to the joist
spacings and of a thickness equal to joist depth. The insulation to
be laid between the joists and to finish over wall plate on
external walls and pushed up tight to Knauf Breatheline.
Polyfoam Supadeck Total, thickness ......mm to be laid over the
Crown Loft Roll at right angles to the ceiling joists. Begin laying
along the full length of one edge of the storage deck. Fix each
Polyfoam Supadeck Total board to the ceiling joist below in the
centre using a 5mm x 150mm screw fixing. All chipboard joints to be
glued.
Any gaps between the edge of the Polyfoam Supadeck Total and
the Knauf Breatheline to be filled with Crown Loft Roll.
Alternatively, refer to NBS clauses: P10/120, 130 and 320